Magnetic field pulsed laser deposition of thin films

ABSTRACT

The present invention relies upon a free space magnetic field in a pulsed laser deposition (PLD) chamber for forming high quality thin films made from diverted ions from a plume evaporated from an ablated target illuminated by a pulsed laser beam. The magnetic field exerts a qv×B Lorentz force upon the ions that is orthogonal to the magnetic field and to their direction of travel in the plume, and curves the ions toward the substrate, while neutral particulates continue to pass by the substrate so that the large neutral particulates are not deposited on the substrate. A shield prevents the deposition of plume species in direct line of sight between the target and the substrate so that only charged ions curved by the magnet are deposited on the substrate. A permanent magnet is used to separate charged species from neutral species. The magnetic field deflects the charged species away from the primary direction of travel of the plume and toward the substrate for deposition of the charged ion species on the substrate. The method provides particulate-free films having improved crystallinity, uniformity and adhesion.

STATEMENT OF GOVERNMENT INTEREST

The invention was made with Government support under Contract No.FO4701-93-C-0094 by the Department of the Air Force. The Government hascertain rights in the invention. The invention described herein may bemanufactured and used by and for the government of the United States forgovernmental purpose without payment of royalty therefor.

SPECIFICATION

1. Field of Invention

The present invention relates to processes for forming thin films. Moreparticularly, the present invention relates to pulsed laser depositionmethods using a desired target material for forming high quality thinfilms on a substrate.

2. Background of the Invention

Pulsed laser deposition (PLD) is a versatile deposition technique thathas been in use for several years. It is based upon the evaporation of atarget material by a high power laser, with its subsequent collection ona desired substrate forming a film on the substrate. The PLD method hasmany advantages including the ease of deposition, and the formation ofcrystalline films with good adhesion at low temperatures, even as low asroom temperature. Another advantage of the PLD technique is the abilityto reproduce the stoichiometry of the target in the film, including thatof multi-component targets. PLD is desirable for routine deposition atroom or higher temperatures providing high quality crystalline thinfilms.

Different methods have been used to reduce the problem of producingclean, particulate-free thin films of any material of choice using PLD.This method is typically used in superconductor film growth processesand other coating processes for forming high quality thin films. PLDinvolves laser ablation and evaporation of a target material by a highpower laser. The ablated material forms a plume comprising bothundesirable large neutral particulates and desirable atoms and ions allof which get deposited on a substrate. The plume extends in a directionoutward from the target. Typically, the substrate is held directly infront of the target, at a distance of a few inches. Examples of variouspulsed laser deposition methods include those taught by Takano, et. al.,U.S. Pat. No. 5,212,151, Schultheiss, et. al. U.S. Pat. No. 5,028,584,Venkatesan, et. al., U.S. Pat. No. 5,015,492, and Owen, U.S. Pat. No.5,614,114.

However, the PLD technique has one continuing problem that has beenextremely difficult to solve, namely the inclusion of undesirableparticulates, typically 0.1 to 5.0 μm in size, which disadvantageouslylimits PLD commercialization. Conventional PLD methods disadvantageouslyproduce about 400 particles cm² /Å particle density. Typically, laserablation of the target results in the creation of charged and neutralspecies in many different sizes. Only species of atomic dimensions ofthe target material are desired to be deposited on the substrate. Iflarge sized particulates form on the substrate, they limit theuniformity of the deposited thin film and its applications. The originof these particulates is thought to be multifaceted. Factors includeprotruding surfaces, craters, micro-cracks in the target that aremechanically dislodged due to laser-induced thermal or mechanical shock,rapid expulsion of trapped gas bubbles beneath the target surface duringlaser irradiation, and the splashing of molten layers of targetmaterial.

For most applications, the creation of large particulates poses aserious problem. For tribological applications, it is desirable todeposit coatings with very high hardness on precision bearings. Thesehard coatings can protect the steel surfaces of bearings from wear, andthereby improve the bearing lifetime. This can be extremely valuable inimproving the performance of moving mechanical assemblies in a varietyof applications such as machinery, pumps etc. PLD is an ideal techniquefor depositing such hard coatings, however, the incorporation of hardparticulates in the coatings can be detrimental to the bearings.Particulates, especially those of a hard material, can be abrasive anddestroy the coating, leading to the production of more debris, andeventually to loss of coating adhesion. The loss of the coating adhesionis highly undesirable. If the coated bearing is used in conjunction witha liquid lubricant, the particulates and debris can also impede thesmooth flow of lubricant through a bearing assembly, consequentlyleading to failure.

The inclusion of particulates during PLD poses a big problem whenmaterials for high performance optical, electronic andmicroelectromechanical systems (MEMS) are deposited. In general, forthese applications, stringent constraints exist for surface smoothness,therefore the tolerance to particulate density and size is generallylow. In particular, for multilayer device fabrication, the presence oflarge particulates that get permanently implanted into the film canlimit the resolution, size, and functionality of features that are to befabricated.

A number of attempts have been made to reduce the particulate density ina PLD film. These include the use of an off-axis method where thesubstrate is positioned parallel to the plume, a velocity filter methodto selectively allow particulates of lower mass, and the dualoverlapping laser method. Each of these methods has certainshortcomings.

The off-axis method is taught by Cheung et. al. in U.S. Pat. No.5,411,772. Cheung teaches a deposition configuration which places asubstrate parallel to the propagation direction of the plume which isproduced when the laser beam impinges upon a target. The substratedisadvantageously extends parallel to the plume limiting film growth.Material deposition on the substrate can only occur from species in theplume that have a significant velocity component perpendicular to theplume propagation direction. This method disadvantageously relies on thepresence of a background gas having a pressure range of 0.1 to 0.001Torr. The role of the background gas is to cause collisions with theablated species. Lighter particulates such as atoms and ions can bescattered toward the substrate through collisions, while heavyparticulates do not experience a significant lateral diffusion andtherefore proceed along their original paths without depositing on thesubstrate. The necessity for random collisions disadvantageously limitsthe crystallinity of the films produced at low temperatures, andrestricts the growth rate seriously.

The velocity filter method is for eliminating large particulates andrelies on the fact that large particles tend to have lower velocitiesthan particles of atomic dimensions. The method uses a filter that willonly allow species having a predetermined velocity to pass through thisfilter and deposit on a substrate. This method has been suggested byAkihama, U.S. Pat. No. 5,126,165. Akihama suggested an apparatus and amethod for depositing material of a predetermined velocity. Directionselection is made with a plate having an aperture that selects materialin the direction of 0.022 steradian with the normal to the targetsurface. Velocity selection is done using a chopper through whichmaterial is allowed to pass only for a predetermined time. In addition,a predetermined dc voltage is applied between the target and substrateto control the spatial and time distributions of charged particulatesarriving at the substrate. The velocity method disadvantageously reliesupon differing species velocities which may not completely separatedesirable ions from undesirable particulates. The velocity filter methodhas also been used to measure the velocity distribution of micron-sizeparticulates, as well as atomic and ionic species present in the laserablation plume. Particulate velocities have been found to be on theorder of a few hundred meters per second while those of atomic and ionicspecies were an order of magnitude larger. This suggests that it wouldbe possible to use velocity selection of species in the plume to preventlarge particulates from depositing on a substrate. The velocity filtermethod has also been adapted to use an electronically actuated shutter,placed between the target and substrate. If a velocity filter is toprevent the entire velocity distribution of large particles fromreaching the substrate, a significant fraction of atomic size particleswill also be filtered out and the growth rate of the film willdisadvantageously drop to almost zero.

U.S. Pat. No. 5,660,746 teaches the dual laser method for forming a filmwith reduced particulate density using a dual-laser deposition process.The preferred embodiment includes the spatial overlap of two laserpulses on a target, with the lasers being of different wavelengths. Thefirst laser irradiates the target surface to form a molten layer, whilethe second laser generates a plume from the molten layer. The two pulseshave a predetermined delay with respect to each other so as to controlthe ejection and subsequent deposition of particulates on the substrate.The dual laser method disadvantageously requires the use of two lasershaving temporal delays when the first CO₂ laser forms a molten layer onthe target and the second UV laser vaporizes the molten target.

A PLD magnetic duct method uses a magnetic duct positioned in thedeposition chamber to drag ions along the field lines of a weak curvedmagnetic field which is uniform along the curved ion path. The methodteaches to provide magnetic fields sufficient to "magnetize" and affectthe direction of flight of electrons in the plasma. The magnetic fieldstrength is disadvantageously not sufficient to magnetize the ions usedfor thin film formation. The method suggests that when the electrons aremagnetized and the ions are not, the ions tend to drift towards theouter wall of the duct, and not be deposited on a substrate. Jordan etal's magnetic duct PLD method teaches the use of magnetic duct to createa uniform magnetic field to guide electron species along the field linestoward the substrate, and disadvantageously does not apply asufficiently strong force upon the ablated ions to guide them toward thesubstrate for the creation of quality thin films.

The predominant problem with PLD methods is the creation and depositionof large particulates that impose a limitation on the applications.Despite numerous advantages of the PLD method, its commercialization hasbeen slow, primarily because of the particulate problem, that has beendifficult to solve. These and other disadvantages or problems are solvedor reduced using the present invention.

SUMMARY OF THE INVENTION

An object of the present invention is to form high quality thin filmsusing a pulsed laser deposition (PLD) chamber wherein a laser beamablates a target material creating a plume of ions diverted anddeposited onto a substrate using a magnetic field.

Another object of the present invention is to form high quality thinfilms using a pulsed laser deposition chamber wherein a laser beamablates a target material creating a plume of ions diverted anddeposited onto a substrate using a magnetic field while preventingdeposition of plume species ejected from the target in a direct line ofsight between the target and substrate.

Another object of the present invention is to form high quality thinfilms using a pulsed laser deposition chamber wherein a laser beamablates a target material creating a plume of ions diverted anddeposited onto a substrate using a magnetic field while preventingdeposition of plume species ejected from the target in a direct line ofsight between the target and substrate, and while not diverting neutralor large charged particles so as to prevent the deposition on thesubstrate of neutral or large charged particles ejected from the target.

The present invention relies upon a free space magnetic field in apulsed laser deposition chamber in which ions from the plume arediverted according to the v×B right hand rule. The magnetic field exertsa Lorentz force upon the ions to deflect them orthogonal to both theirinitial direction of travel and to the magnetic field, toward thesubstrate, while neutral particulates continue unaffected past thesubstrate. A preferred permanent magnet is used to separate chargedspecies present in the laser ablation plume from neutral species, thattypically include the undesirable particulates. The magnetic fielddeflects charged species from the primary direction of travel of theplume by means of a (qv×B) Lorentz force which does not affect neutralspecies. The deflection occurs in free space without the use of amagnetic conduit. The magnetic field deflects the charged species awayfrom the primary direction of travel of the plume and toward thesubstrate for deposition of the charged ion species on the substrate.The desired substrate is positioned such that it collects the deflectedcharged species to produce a thin film. The neutral species of the plumethat typically include the undesired heavy and large particulatesexperience no effect of the magnetic field and proceed undeflected alonga straight path. A simple mechanical shield is positioned in the directline of sight between the target and the substrate to prevent anyparticulates in direct line of sight between the target and substratefrom reaching the substrate. The substrate can be biased to increase theion yield at its surface. The substrate can be held at room temperatureor at any other elevated temperature using a substrate heater. Theresulting film on the substrate is essentially free of particulates. Thecombination of the magnetic field and a bias applied to the substrateoffers the advantage of reducing particulate densities without asignificant reduction in film growth rate.

The present invention does not require a parallel geometry as does thesubstrate for off-axis deposition, does not use mechanical velocityselections of various particulates in the plume, does not use duallasers, but rather uses a single laser and a single magnet. The presentinvention produces extremely clean films with highly reduced particulatedensities and size, and having appreciable deposition rates. This methodfavors useful film properties, such as crystallinity and good adhesion,even at room temperature, because it relies upon using high energy ionsfor the deposition. The method therefore has tremendous potential forapplications where the substrate is thermally sensitive.

The invention is relatively inexpensive and very simple to implementusing conventional components while being insensitive to any depositionon the magnet itself, since this does not alter the magnetic field withcontinued use of the magnet inside the deposition chamber. A standardPLD system can be modified with a magnet and a substrate shield, withoutthe use of a background gas and without the added costs andcomplications of a gas delivery system and pressure regulation. Thepresent invention may be applied to the production of a film of anymaterial. These and other advantages will become more apparent from thefollowing detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a magnetic field pulsed laser depositionsystem.

FIG. 2 is a block diagram depicting ion deflection in the magnetic fieldpulsed laser deposition system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the FIGS. 1 and 2, the magnetic field pulsed laserdeposition (PLD) system is used for obtaining highly uniform and smoothfilms with the PLD process. An excimer laser operating with KrF at awavelength of 248 nm, and having a pulse width that is typically 30 nsprovides a laser beam. The laser beam is focused on the target ofchoice, held in a target holder inside a vacuum chamber, with a lenspositioned outside the chamber. Typical laser fluences are 1-15 J/cm².The vacuum chamber is maintained at a base pressure of 1×10⁻⁷ to 1×10⁻⁸torr and should be less than 10⁻⁴ torr. The substrate of choice isplaced in a substrate holder inside the vacuum chamber. The substratecan be any solid material which is appropriate because of itsmechanical, optical, electronic, or chemical properties. The target ispositioned in a conventional target holder, that is rotated at a speedof approximately 1000 RPM during deposition. The target can be anysolid, laser-ablatable material. In the exemplar configuration, a targetof a material is used to deposit a film of the same material. A reactivegas or buffer could be introduced into the vacuum chamber that reactswith the plasma plume forming a new material which is still ionized andwill still be deflected towards the substrate. A constituent specie ofthe target will nonetheless still be deposited on the substrate. Forexample, a nitrogen reactive gas and a TiC target may produce ionizedTiCN that is deflected and deposited. For another example, a methane gasCH₄ with a Ti target can be used to deposit TiC films. The target isrotated by a target holder, while being exposed to the pulsed laserbeam, for uniform ablation. The target holder is positioned at a fortyfive degree angle with respect to the incoming laser beam. The substrateholder can be a rotating substrate holder, for example at 1000 RPM, forimproved uniformity of the deposited film. The substrate can beelectrically biased, for example from zero to -30 volt DC for improvingthe rate of film growth. The substrate can be heated to a hightemperature, for example up to 700° C., but can also be maintained atroom temperature 25° C. A permanent magnet with a field strength of1000-4000 Gauss at the poles and 400-700 Gauss between the poles ispositioned inside the vacuum chamber, approximately three to four inchesfrom the target. The magnet is oriented such that the plume extends intothe region between the two poles. The magnet is positioned such that theheight of magnet is at the center of the target. The magnetic field canbe generated using a variety of magnetic means, such as a permanentmagnet, an electromagnet or a quadrapole with an exemplar range of 100to 10,000 Gauss.

The lens may be a focusing means where the focused laser light canprovide a sufficient power density to generate a laser induced plasmaplume from the target. When the laser beam is focused upon the target,the ablation plume is initially directed in part toward the magnet. Ionsin the plume are deflected upwards as the ions experience the magneticfield at the poles. The substrate is held face down in a substrateholder positioned horizontally above the magnet. The substrate ispositioned slightly in front of the north pole of the magnet, where themagnetic field and therefore the upward ion plume deflection is thestrongest. Conventional substrate cleaning involves severalultrasonications in heptane, followed by drying in a stream of drynitrogen gas. Prior to loading in the vacuum chamber, any organicresidue or other contamination on the substrate surface is removed bylaser ablation using the same excimer laser. The substrate is maintainedat room temperature, and can be heated if necessary in to improve filmadhesion and/or crystallinity. The substrate can be biased, floated, orheld at ground potential.

A small shield preferably consisting of a copper plate of one by threeinches, is lined up in front of the substrate, such that it shields thesubstrate from any particulates that can reach the substrate from thetarget through a direct line-of-sight trajectory between the target andsubstrate.

Using this magnetic field PLD system, extremely clean and uniformcoatings of polycrystalline TiC have been obtained on 52100 bearingsteel flats and races at room temperature. Following deposition, thefilm morphology can be observed with a scanning electron microscope forverification of films growth and quality. The resulting TiC films areharder than steel and are polycrystalline even when deposited at roomtemperature. This magnetic field PLD system eliminates all particulateswith average sizes greater than one half micron as an improvement overthe conventional PLD processes that has a high density of particlessizes from one to five microns. The particulate density using thepresent invention is less than 10/cm² /Å which is a factor of fortylower than for a standard line-of-sight PLD processes. Exemplarsubstrates include 52100 steel, 440C steel, REX20 steel, silicon,silicon nitride, silicon dioxide, copper or aluminum.

The magnetic field PLD system demonstrates particulate reduction withoutsacrificing film growth rate. Particulates larger than one half micronsare eliminated, and extremely low densities of less than 10/cm² /Å areobserved for particulates smaller than one half micron in size. Thegrowth rate is high at 0.6 μm per hour with the laser operating at fiftyHertz. The preferred magnetic field PLD system can easily be scaled upby using higher laser repetition rates available in many commerciallaser sources. The magnetic field PLD system not only provides uniformfilms having no large particulates, but also tends to create desirablecrystalline films having good adhesion to the substrate.

The magnetic field PLD system uses a permanent magnet to deflect ablatedions to achieve uniform films. The magnet may be positioned anywhere infront of the plume. The deflection of the plume is clearly visible,therefore positioning the substrate is straightforward. A separatemechanical shield prevents particles from hitting the substrate. Inaddition to using a permanent magnet, electromagnets or a quadrapole mayalso be used. Those skilled in the art of PLD can make enhancements andimprovements to the preferred magnetic field PLD system and method, butthose improvements and enhancements may nonetheless fall within thespirit and scope of the following claims.

What is claimed is:
 1. A method of depositing a thin film on asubstrate, the method comprising the steps of,ablating a target with alaser beam creating a plume having charged species and neutral species,shielding the substrate to prevent species of the plume from travelingfrom the target to the substrate through a direct line of sight betweenthe target and the substrate, and deflecting by a magnetic field thecharged species in the plume towards the substrate to deposit thecharged species on the substrate, the deflection being orthogonal to theplume and to the magnetic field, the deflection is an orthogonal v×BLorentz magnetic deflection with the magnetic field having a vector B,the charged species having a velocity vector v, and the deflection beingorthogonal to both vectors v and B to orthogonally deflect the chargedspecies around the shielding and towards the substrate.
 2. The method ofclaim 1 whereinthe target and thin film consist of titanium carbide, andthe substrate is made of steel.
 3. The method of claim 1 furthercomprising the step offocusing the laser beam onto the target.
 4. Themethod of claim 1 wherein the ablating step comprises the stepsofrotating the target to periodically expose a surface of the target tothe laser beam to ablate the surface of the target to create the plume,and focusing the laser beam onto the target.
 5. The method of claim 1wherein the laser beam is a pulsed laser beam.